Consumables Corner: Welding heat treatable steels
Q: Our shop won a contract that requires welding 4130 steel. I’ve been told this is a heat-treatable steel. Can you tell me which different welding procedures have to be used to weld this material?
A: The most commonly welded heat-treatable steels are 4130, 4340, and 8630. However, any steel prefaced with a 41, 43, or 86 is considered heat-treatable. When a weldment is heat treated, it is put into a furnace at a predetermined temperature and cooling rate. The heat treatment produces a desired microstructure that changes the mechanical properties of the material to a desired strength and ductility level. Do not confuse a heat treatment with postweld stress relief. The postweld stress relief is exactly that—a method to relieve stresses that are the direct result of the welding process.
To select the correct filler metal you must ask yourself the follow questions: Will the weldment be heat treated after welding or used in the as-welded condition? Are all the weld joints to be joined to the same-strength material?
If the parts are going to be heat treated, you must weld them with a similarly heat-treated filler metal to ensure that the weld metal will respond to the heat treatment consistently with the parent metal. This ensures that the weld metal will have strength and ductility similar to the base material after heat treatment.
If the welded component will be used in the as-welded or stress-relieved condition, select a low-hydrogen type electrode with a tensile strength of about 110,000 PSI. In some cases, a specification will allow or require a lower-strength filler material. On single pass fillet welds, tests have shown that diluting the weld and base material will produce an adequate strength. When a higher-strength part is welded to a lower-strength carbon steel, the filler metal should be a low-hydrogen type that matches the lower-strength material.
To prevent major welding problems when welding heat-treatable steel, you must use proper preheat and interpass temperatures to eliminate or reduce cracking. Cracks can occur in the HAZ of the material or possibly the centerline in the weld deposit. The proper preheat and interpass temperature is usually 400 to 600 degrees F for heat-treatable steels.
When welding a quench and tempered steel like T1, which has received a heat treatment to achieve a particular strength level, use much lower preheat and interpass temperatures to help control heat input so the designed strength level is maintained. Usually for T1 steels less than 1 in. thick, a 50-degree-F preheat is recommended. As thickness increases, preheat can increase: for 1 to 2 in., preheat to 150 degrees; for more than 2 in., preheat to 200 degrees.
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